FIG. 1 shows a structural schematic diagram of an Evolved Universal Mobile Telecommunication System Radio Access Network (E-UTRAN for short) of the Long Term Evolution (LTE for short) system according to the prior art. As shown in FIG. 1, the LTE consists of the E-UTRAN and an Evolved Packet Core (EPC for short), with the network having a flat structure. The E-UTRAN is connected with the EPC via an S1 interface, wherein the E-UTRAN consists of a plurality of Evolved NodeBs (eNB for short) which are connected to each other via the X2 interface; and the EPC consists of a Mobility Management Entity (MME for short) and a Serving Gateway Entity (S-GW for short).
In the LET network system, when User Equipment (UE for short) moves in the network coverage area, such movement might cause variation of the serving cell of the UE. In order to keep the service continuity of the UE, it is required that the network assists the UE to handover among individual cells, wherein an initialization side of the handover is called a source side while a destination side of the handover is called a target side.
To satisfy the demand of on higher bandwidth, the LTE-Advanced (LTE-A for short) proposes a method of carrier aggregation to achieve higher bandwidth, in which a UE can operate at a plurality of continuous or discontinuous carries (each carrier is called a Component Carrier, that is, CC for short). Among the CCs, there is one Primary Component Carrier (PCC for short), while the others are Secondary Component Carriers (SCC for short). For each CC, there is a serving cell providing service for the UE, wherein the serving cell on the PCC is called a Primary Cell (Pcell for short), while the serving cell on the SCC is called a Secondary Cell (Scell for short). Radio resource of the LTE-A can be assigned to more than one carrier, that is, at least one Pcell and zero to several Scells.
In a LTE-A multi-carrier system in the prior art, when a plurality of cells can be treated as the target cell, a source base station can not determine which cell should be used as the target cell of the handover, therefore the handover can not be achieved.